The present invention relates to the field of reflective color liquid crystal display devices.
A reflective liquid crystal display device (reflective LCD) employs a liquid crystal panel to modulate ambient light entering from the front panel surface and to reflect it on a reflector provided on the rear face of the panel for display. The reflective LCD thus eliminates the need of a backlight which is necessary for a transmissive liquid crystal display device (transmissive LCD). This enables the reduction of power consumption, making the reflective LCD suitable for personal digital assistants and other mobile equipment.
However, since the reflective LCD displays images by reflecting ambient light, there is no function for adjusting the incident light level. Accordingly, if the luminance of ambient light is weak, such as when the equipment is used indoors or at night, the display screen becomes extremely dark due to low ambient light, with the consequent disadvantage of degraded viewability. The reflective LCD thus needs a high degree of reflectance to efficiently reflect the entering ambient light.
One approach to increase the reflectance includes the prevention of optical transmittance loss in the liquid crystal cells and optical members, combined with the increase of reflectance by the use of a reflector. By focusing on an optical transmission loss in a polarizer, methods of reducing optical transmission loss in the liquid crystal cells and optical members include the guest-host display system (Japanese Laid-open Patent No. H7-146469) which eliminates the use of a polarizer, and the single-polarizer system (Japanese Laid-open Patent No. H7-84252) that uses only one polarizer.
A method of increasing the reflectance of the reflector include a system for forming a reflective electrode (Japanese Laid-open Patent No. H8101384), which involves disposing the reflector, conventionally provided outside the liquid crystal cell, inside the liquid crystal cell, and to use aluminum, which has greater reflectance and lower electric resistance, as the material for the reflector to create a combined reflector and electrode. Another method is a system using a liquid crystal cell, a retardation plate, and a polarizer, with a light scattering function effected by concavity and convexity on the reflective electrode surface (Japanese Laid-open Patent No. H6-167708).
For example, in the reflective LCD shown in FIG. 9, a liquid crystal layer 7 is interposed between a glass substrate 1 on which a reflective electrode 2 is deposited, and an opposing glass substrate 6 on which red, green, and blue color filters 3a, 3b, and 3c, and a transparent electrode 5 are formed. A retardation plate 9 and polarizer 10 are disposed on the outer face of the glass substrate 6. A light-blocking layer 4 is disposed at the gap between the color filter layers to prevent light leakage. This type of reflective LCD employs both the single polarizer system using one polarizer and the system of providing the reflective electrode 2 having concavity and convexity inside the liquid crystal cell. The scattering performance is added to the reflective electrode with the aim of improving viewability by increasing the diffuse reflection. The incident light passes through the polarizer 10, and becomes linearly polarized light. This light is modulated by the retardation plate 9 and the liquid crystal layer 7, reflected on the surface of the reflective electrode 2, and then reaches the polarizer 10 after passing back through the liquid crystal layer 7 and retardation plate 9.
For displaying white (bright) and black (dark) colors in the reflective LCD using the single polarizer system, the light reflected on the reflective electrode face requires to be circularly polarized light over the entire visible light range in the case of black (dark) display, and linearly polarized light in the case of white (bright) display. In order to satisfy these conditions, the phase difference between the ordinary light and extraordinary light when the light passes in both ways to and from the retardation plate and liquid crystal in the visible range may need to satisfy the following formula in the case of white display:
2xcfx80*(RL+RF)/xcex=xcfx80*m;
in which
RL: Retardation of the liquid crystal layer,
xcex: Wavelength of the light,
RF: Retardation of the retardation plate, and
m: Natural number.
For black display, the phase difference may need to satisfy the following formula:
2xcfx80*(RL+RF)/xcex=xcfx80*(m-{fraction (1/2)}).
Here, 2xcfx80/xcex times of retardation is the phase difference.
If the liquid crystal layer has a homogeneous orientation, the retardation RL of the liquid crystal layer may be expressed using Formula xcex94n*d, where xcex94n is the refractive index anisotropy of liquid crystal, and d is the thickness of the liquid crystal layer.
However, in the above reflective LCD, the light passing each dot of red, green, and blue configuring pixels has a different optical wavelength, and thus the above formula may not be satisfied over the entire visible range. For example, if xcex94n, d, and RF are determined to satisfy the formula for green light, which has the highest visibility (around 550 nm), the same formula is not satisfied in other wavelength ranges for blue, which has a shorter wavelength (around 450 nm) and red, which has a longer wavelength (around 650 nm). In addition, since xcex94n and RF are to some degree dependent on wavelength, the above formula is even more difficult to satisfy. Therefore, the light is insufficiently blocked in the black display, causing loss of contrast. Or, coloring (in particular, yellowing) due to degraded light modulation rate in the blue and red dots in the liquid crystal layer in white and halftone displays may occur.
The present invention aims to offer a reflective color LCD which prevents low contrast and coloring.
The reflective color LCD of the present invention includes a first substrate; a second substrate; a liquid crystal layer interposed between the first and second substrates; a reflective layer formed on an inner face of the first substrate, a color filter layer formed on an inner face of one of the first and second substrates, the color filter corresponding to each of red, green, and blue dots; a polarizer disposed on an outer face of the second substrate; and one of (1) a retardation plate disposed on the outer face of the second substrate, (2) an retardation layer formed on the inner face of the second substrate, and (3) a retardation layer formed on the inner face of the first substrate; wherein the reflective color liquid crystal display device is configured to satisfy Formulae 3 and 4:
0.9xe2x89xa6((xcex1*dR*xcex94nR+ReR)/xcexR)/((xcex1*dG*xcex94nG+ReG)/xcexG)xe2x89xa61.1xe2x80x83xe2x80x83(3); and
0.9xe2x89xa6((xcex1*dB*xcex94nB+ReB)/xcexB)/((xcex1*dG*xcex94nG+ReG)/xcexG)xe2x89xa61.1xe2x80x83xe2x80x83(4); and
in which, dR, dG, and dB: Thickness of the liquid crystal layer at each of the red, green, and blue dots configuring a pixel; xcexR, xcexG, and xcexB: Wavelength of visible light passing each dot; xcex94nR, xcex94nG, and xcex94nB: Refractive index anisotropy of the liquid crystal layer when the wavelengths of visible light are xcexR, xcexG, and xcexB; ReR, ReG, and ReB: Retardation of one of the retardation plate and the retardation layer; and xcex1: coefficient dependent on a twisting angle of liquid crystal molecules in the liquid crystal layer, the coefficient being xcex1=1 when the twisting angle is 0 and xcex1=0.69 when the twisting angle is 45xc2x0.
Accordingly, the color reflective LCD of the single polarizer system prevents low contrast and reduces coloring in white and halftone displays by specifying phase difference of the light passing each of red, green, and blue dots to a predetermined range.